Inverse feed-back amplifier



Filed Nov. 15, 1959 JAAAA 8 1 I .INVENTOR oomwauuosAr I yg-Z ATTORNEY May 12, 1%22 UNITED STATES PAT OFFICE INVERSE FEED-BACK AMPLIFIER of New South W ales, Australia Application November 15, 1939, Serial No.

In Australia November 15, 1938 6 Claims. (01. 179-17 1) This invention relates to inverse feed-back amplifiers and has particular reference to amplifier circuit arrangements in which inverse feedback is employed to counteract distortion.

With amplifier equipment, particularly for broadcasting purposes, it is often desired to op-' erate separate amplifier output channels from a common input source. Such an arrangement is commonly used when splitting a programme between several lines, separates output valves being used for each line to obviate a complete break in the programme should one line short or become noisy.

In addition, and for similar reasons, separate channels are often desirable in programme amplifiers, for feeding the monitoring system and the mainprogramme circuit. To compensate for non-linear distortions in the amplifying characteristics, it is highly desirable that such arrangements be available for use with inverse feedback circuits; but the usual circuits of this type introduce two serious difficulties. If the feedback is derived from a portion of the plate circuit of each of, say, three stages, and fed back to the input of the same stage, or further back if desired, an E. M. F. impressed on the. output terminals of one stage appears in both the others not greatly reduced in magnitude. This means of course that noise on one output linecan appear at the output of the other. channels, and

in general the channels are effectively coupled] together by the inclusion of such feedback. In addition, variation of load on one output, from open circuit conditions to the limiting case of a short circuit, alters the total amount of feedback, and consequently the overall gain, by appreciable amounts.

,Both of the foregoing effects vary with the amount of feedback and the method of applying it and it is difiicult to obtain a worthwhile amount of inverse feedback by known methods.

without introducing the aforesaid effects.

The principal object of the present invention is, therefore, to provide an improved method of deriving feedback potentials for use ininverse feedback circuit arrangements, which permits a large amount of feedback to be obtained in a simple manner withv excellent stability, and which is not subject to the foregoing undesirable characteristics. a v

A further object of the invention is to provide a method of deriving feedback potentials which has certain advantageous features in normal inverse feedback applications, in that undesirable phase shift is reduced.

A still further object of the invention is to provide an improved method of obtaining feedback potentials foruse in connection with inverse feedback circuit arrangements employed in multiple output amplifiers.

In accordance' with this-invention: feedback potentials for' acircuit arrangement employing inverse feedback are derived'from across a load impedance in the screen grid circuit of a multiple gridoutput' valve'or valves.-

The invention is now described and illustrated in connection with the accompanying drawing.

In the said drawing 1 I i Figure '1 r is a circuit diagram showing one method of deriving feedback potentials in accordance with this invention, and

Figure 2 shows a method of applying the in-' vention-to an amplifier having multiple output stages. l r

Referring to 'Figure 1, the output stage of an amplifier is shown as employing a pentode valve 5 having a cathode l9, a control grid20, a screen grid 2|, a suppressor grid- 22 and an anode 23.

Alternating potentials which are to be ampli:

fied'arefed to the input terminals ll, l8, and

applied to 'the grid 20 through the medium of coupling condenser l5 and 'acrossthe grid re-"v sistor l6. The grid20is given its desired potential bias by means of a so-called self-biasing re-' sistor 3 connected between cathode la' and round. Those skilled in the art are wellacquainted with this method of obtaining. a suit-- able grid biasingpotential and hence it will'be.

readily understood' that the bias is obtained due to the voltage drop across resistor 3 when current flows through the anode-cathode circuit.

The voltage thus developed across the resistor 3 is applied to the grid320 through'the grid re- 1 sistor l6. A by-pass condenser 4 isconnected' in shunt across resistor 3 in order to" provide. a; low impedance path to ground for alternating currents of relatively .high frequency flowing in the cathode circuit. Although the self-biasing methodhas been described. for obtainingbiasing potentials for the grid, it is to be clearly understood that any well known method of applying the desired'biasing potentials to grid, 20 may be employed without afiecting the scope of this invention.

Operating potentials are applied to the anode The required voltage is applied to the screen grid 2| from the aforesaid positive terminal'25 through the filter network 8,- 9, and the seriesconnected impedance 1. The decoupling impedance 8 maybe an inductance, or a resistanceas.

. shown. 7

, Feedback potentials are-obtained in accord-" 1 ance with this invention from a potential divider comprising condenser II and resistors l0, [4, concan be normal for components usually associated I with" this type of circuit. Components I, 8, 8, I0, II and I4 are associated with the feedback circuit and their relative values are controlled by the considerations hereinafter explained.- I

The decoupling impedance 8-provides isolation of the A. C, potentials on the screen fromthe normal high voltage supply circuit, and where 5. frequency compensated feedback is required an impedance value is selected for 8 which enables the condenser 9 to be of such a value that the feedback network is compensated suitably at low frequencies, as it will be apparent thatthe impedance of 9 is actually a portion of the load impedance in the screencircuit. 1

In an experimental assembly where 5 was a type 6F6G pentode valve and I5, I6, 3, 4 and 5 of usual values, the decoupling resistor 8-was 10,000 ohms. However, although 10,000 ohms was the actual value adopted, the value is. not critical, and any value between 5,000 and 20,000 ohms may be employed withoutseriously affecting the operation of the invention.

In order to keep phase shift at a minimum and also to maintain the usual operating potentials and conditions in asimple manner, resistor I should be of a low value. Wherefeedback is arranged over an amplifier of reasonably high gain;

the value of the component I may be between 100 and 2,000 ohms. In an experimental application a value of 1,000ohms was used for 1 and 12 microfarads for 9. Between certain limits dependent on other components, increase in the capacity'value of 9 decreases the feedback at lowfrequencies compared with that at higher frequencies.

Condenser I I is necessary in the example shown in ,Figure 1 to block the high voltage potential from other portions of the. feedback network and is also effective in compensating the feedback at low frequencies. denser IIv decreases'the feedbackat low frequencies, and therefore relative values of the components 9 and II can be adjusted to obtain ade--' quate low frequency compensation. The actual when applying feedback to an input transformer of a previous stage, as in Figure 2, were as follows:

Ohms Resistor I 15,000 Resistor I2 1,000 Resistor I4 2,000

The condenser I3 connected in shunt'across the resistor I2 provides high frequency compen-' sation in the feedback path.

Whilst an improved method has been described of deriving feedback potentials by the use of a" potential divider across-the load impedance in the screen circuit of a pentode valve, it will be Decrease in the value of con- I realised that the feedback network itself can take any of the usual forms known in the art, and that the circuit and values of Figure 1 are explained for the purposes of illustration only and represent practical arrangements used with the particular application for which the whole circuit arrangement was developed. Inthe application of the invention to an amplifier feeding a plurality of output channels, as exemplified by Figure 2, the method disclosed by Figure l is slightly'modified.

In this modified arrangement a single stage amplifying valve 38 is shown with its grid coupled to the input terminals 4I through the transformer 15 40, and. its anode coupled to, the grids of the v parallel connected-output-valves 5, 5a through theresistance capacity coupling network 32, I5,

I6. The anodes of the parallel output valves 05, 5a arecoupled to separate utilization means through'the output coupling'transformers 6 and 6a respectively.

- Operating potentials are supplied to the anodes of the'valves 5, 5a from the positive terminal I H. T. {ofthe supply source (not shown) through the impedances 28; 29; and to the anode of valve 38 through the decoupling-network 3 I, 33 and the anode resistor 32. Potentials for the screen grid of valve 38 are taken from 'a common connection I between resistors 3|, '32 through a dropping resistor34.-jThe by pa'ss condenser maintains the screen-at cathode'potentialfor the alternating. potentials which are'to be amplified. Biasing potentials are supplied to the grids of the separate valves"-th'rough self-biasing networks in the individual cathode leads.

The feedback arrangement and its operation are substantially the same as described in connection with Fig. 1 and, as like reference numer als have been used to indicate like parts, a further description is considered unnecessary.

The feedback load, I, B and 9 is shown in Fig. 2 common to both screens as it was. found that no undesirable effects were thus "created.

Feedback potentials are taken from'the p0 tential'divider- I I, I0, I4, in the present example and fed to the input of the valve 38 in degenerative sense; through a tertiary winding on the transformer 40- and the frequency compensating network I2,' I3.

The feedback could, of course, be connected through other types of networks to different points in the amplifier by methods well known in the art, and the above is merely one example of its application to a particular experimental arrangement.

back network has been shown as frequency compensated, it isto be understood that it is within the scope of this invention to dispense with the compensation if desired.

I claim:

1. An amplifier circuit arrangement comprising a plurality of electron discharge tube stages, each tube of which includes a cathode, an anode, 5 a controlgrid and a screen grid, said screen grids of stages'followinga first stage being connected in parallel, means for producing inverse feedback potentials on the .control' grid of the tube in the first stage, said means including a load impedance in circuit with said parallel connected screen grids,'and means for rendering said load impedance;frequency-dependent.

2. A circuit arrangement according to claim 1 in which said load impedance is constituted by series-connected resistance elements, and the last said means is constituted by .resistive and capacitive elements of a filter system.

3. A circuit arrangement comprising aplurality of multi-grid electron discharge tubes, one of said tubes being in a first stage and the remaining tube complement being in a second stage, parallel connections to corresponding grids of the second stage tubes, and inverse feed-back means adapted to apply control-energy to the tube of the first stage, said control energy being derived from potential variations on those parallel-connected grids of the second stage which serve as screen grids.

4. A device according to claim 3 and having a frequency-dependent filter included in said feedback means. v 1

5. An amplifier circuit arrangement compris ing a pentode discharge tube in a first stage, a

plurality of parallel connected pentode discharge tubes in a second stage, said second stage tubes having independent output circuits and having screen grids which are parallel-connected to a common load impedance, and means for deriving from said load impedance inverse feed-back potentials for partial control of the tube in the first stage. I

6. An amplifier circuit arrangement comprising two stages of discharge tubes the second stage comprising a plurality of tubes having parallel connected input circuits and a plurality of independent output circuits, each tube having a cathode, an anode, and at least two grids including a control grid and a screen grid, an inverse feed-back circuit connected between the screen grids of the second stage and the control grid of the first stage, frequency dependent elements in said feed-back circuit, a source of operating potential, and a load impedance interposed between said source and the screen grids of the second stage, said load impedance being effective to produce the Wanted feed-back potentials 20 in said feed-back circuit.

1 DONALD GORDON LINDSAY. L 

